1. Field of the Invention
The present invention generally relates to thermal barrier coating systems for components exposed to high temperatures, such as gas turbine engine combustor liners and shrouds. More particularly, this invention is directed to a thermal barrier coating system having a multilayer thermal barrier coating comprising a spallation-resistant inner layer and a phase-stable outer layer.
2. Description of the Related Art
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating topcoat, also referred to as the thermal barrier coating or TBC. Ceramic materials are used as TBC materials because of their high temperature capability and low thermal conductivity. The most common TBC material is zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another alkaline-earth metal oxide, ceria (CeO2) or another rare-earth metal oxide, or mixtures of these oxides. Binary yttria-stabilized zirconia (YSZ) has particularly found wide use as the TBC material on gas turbine engine components because of its low thermal conductivity, high temperature capability including desirable thermal cycle fatigue properties, and relative ease of deposition by thermal spraying (e.g., air plasma spraying (APS) and high-velocity oxygen flame (HVOF) spraying) and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD).
To be effective, TBC's must remain adherent through many heating and cooling cycles. This requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic materials and the superalloys typically used to form turbine engine components. As is known in the art, zirconia is stabilized with the above-noted oxides to inhibit a tetragonal to monoclinic phase transformation at about 1000° C., which results in a 3% to 4% volume change that can cause spallation. At room temperature, the more stable tetragonal phase is obtained and the monoclinic phase is minimized if zirconia is stabilized by at least about six weight percent yttria. A stabilizer (e.g., yttria) content of seventeen weight percent or more ensures a fully stable cubic phase. Though thermal conductivity of YSZ increases with decreasing yttria content, the conventional practice has been to partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) with the understanding that 6-8% YSZ TBC is more adherent and spallation-resistant when subjected to high temperature thermal cycling than YSZ TBC containing greater amounts of yttria, particularly fully stabilized YSZ. Furthermore, partially stabilized YSZ (e.g., 6-8% YSZ) is known to be more erosion-resistant than fully stabilized YSZ (e.g., 20% YSZ).
The spallation resistance of TBC's is further improved with the use of an environmentally-protective metallic bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides. When subjected to an oxidizing environment, these aluminum-rich bond coats develop an aluminum oxide (alumina) scale that is advantageously capable of chemically bonding a ceramic TBC to the bond coat and the underlying substrate.
Further improvements in TBC spallation resistance have been achieved through the development of TBC microstructures that exhibit enhanced strain tolerance as a result of the presence of porosity, vertical microcracks and/or segmentation. Segmentation indicates that the TBC has columnar grain boundaries oriented perpendicular to the surface of the component, such as that achieved with PVD processes, e.g., electron beam physical vapor deposition (EBPVD). The term “vertical microcracks” is used herein to denote fine cracks that are intentionally developed in thermal sprayed TBC's, whose microstructures generally consist of splats of irregular flat (noncolumnar) grains formed by solidification of molten particles of the TBC material. As is known in the art, ceramic TBC's having columnar grains and vertical microcracks are more readily able to expand with the underlying substrate without causing damaging stresses that lead to spallation. Plasma-sprayed TBC's with microcracks are discussed in U.S. Pat. Nos. 5,073,433, 5,520,516, 5,830,586, 5,897,921, 5,989,343 and 6,047,539, and in Sumner et al., “Development of Improved-Durability Plasma Sprayed Ceramic Coatings for Gas Turbine Engines,” AIAA/SAE/ASME 16th Joint Propulsion Conference, Jun. 30 through Jul. 2, 1980, Duvall et al., “Ceramic Thermal Barrier Coatings for Turbine Engine Components,” ASME paper 82-GT-322.
The outer surface of a TBC sustains the highest temperatures, with higher TBC surface temperatures occurring with greater TBC thicknesses. As higher gas turbine operating temperatures are sought to increase engine efficiency, thicker TBC's (e.g., above 500 micrometers) are necessary to protect components from higher flow path gas temperatures. In applications such as the Joint Strategic Fighter (JSF), temperatures within the combustor and high pressure turbine (HPT) shroud may be as high as about 2800° F. (about 1540° C.), which is above the phase transformation temperature for 6-8% YSZ. As such, a 500+ micrometer-thick 6-8% YSZ TBC would be at a higher risk of spallation brought on by phase transformation.
In view of the above, it would be desirable if an improved TBC system were available that was suitable for use in applications where operating temperatures necessitate thick TBC's, resulting in TBC surface temperatures above the phase transformation temperature of partially stabilized YSZ.